The preparation of hydrocarbons from a gaseous mixture comprising carbon monoxide and hydrogen (synthesis gas) by contacting the mixture with a catalyst at elevated temperature and pressure is well known in the art and is commonly referred to as Fischer-Tropsch synthesis.
Catalysts that may be suitably used in a Fischer-Tropsch synthesis process typically contain one or more catalytically active metals from Groups 8 to 10 of the Periodic Table of the Elements. In particular, iron, nickel, cobalt and ruthenium are well known catalytically active metals for such catalyst and may be optionally combined with one or more metal oxides and/or metals as promoters. Cobalt has been found to be the most suitable for catalysing a process in which synthesis gas is converted to primarily paraffinic hydrocarbons containing 5 or more carbon atoms. In other words, the C5+ selectivity of the catalyst is high.
Similar catalyst compositions are also known in other fields including JP-A-404122454 which describes an exhaust gas purification catalyst comprising an active platinum group element such as ruthenium, rubidium, palladium and platinum or a metal oxide such as chromium, manganese, cobalt and nickel on an alumina, silica, titania, silica-titania or alumina-titania carrier. Catalysts of the invention are fitted in an exhaust gas purification catalytic converter and may be used in controlling emissions from gasoline engines.
U.S. Pat. No. 5,134,109 provides a catalyst for the steam reforming of hydrocarbons, which comprises at least one metal from rhodium and ruthenium and at least one metal from cobalt and manganese deposited on a carrier which is preferably zirconia or stabilised zirconia.
JP-A-60064631 discloses a catalyst comprising an iron group metal such as cobalt and iron, a platinum group metal such as ruthenium, rhodium, palladium, platinum and iridium, and manganese oxide, supported on a carrier comprising titanium oxide. JP-A-60064631 further discloses a method for the production of high calorie gas containing hydrocarbons of 1–4 carbons for use as fuels, from low calorie gas containing a mixture of hydrogen, carbon monoxide and optionally carbon dioxide.
JP-A-60064631 is primarily concerned with a method for the production of methane and C2-4 hydrocarbons and does not concern itself in any way with increasing % C5+ selectivity during the conversion of low calorie gas. Indeed, it can seen from Example 2 therein, which is the only example of conversion of a simple CO/H2 mixture, that the treatment of a mixture of 3 parts H2 and 1 part CO in the presence of a catalyst composition comprising 10% Co, 6% Mn2O3 and 2% Ru on a titanium carrier, results in 74.6% CH4, 7.3% C2H6, 5.5% C3H8, 2.6% C4H10 and 10.0% CO2 (by % volume), i.e. the presence of C5+ hydrocarbons was not detected. This conversion was effected at 320° C., and although the broadest temperature range disclosed for the process is 150 to 400° C., it is stated that the preferred range is 260 to 350° C.
Although, U.S. Pat. No. 4,568,663 describes a rhenium-promoted cobalt catalyst on an inorganic oxide support which is preferably titania, which catalyst may be employed in production of hydrocarbons by both FT synthesis and the conversion of methanol, as being highly active, this disclosure is discussed in column 2, lines 19 to 35, of U.S. Pat. No. 4,857,559, and contrasted with the corresponding alumina-supported catalyst, which has significantly higher activity.
Much research effort has been directed to finding catalysts having a higher C5+ selectivity than known catalysts at the same or higher activity.
U.S. Pat. No. 4,857,559 concerns the addition of rhenium to cobalt on a number of common supports including alumina, silica, titania, chromia, magnesia and zirconia and a process for the FT synthesis of hydrocarbons using said catalyst. However, it is recognised therein (e.g. column 4, lines 54 to 59 and column 15, lines 51–55) that whilst supports other than alumina may be used, such supports produce catalysts with much lower activities. It is found in U.S. Pat. No. 4,857,559 that the hydrocarbon yield obtained by the addition of rhenium to alumina-supported cobalt catalyst is greater than the corresponding titania-supported catalyst. In particular, the FT conversion of synthesis gas into hydrocarbons show lower % CH4 selectivity, higher % CO conversion and higher C2+ selectivity for rhenium-promoted cobalt catalysts on alumina, than for similar catalysts on titania (Table 1).
TABLE 1Example% CO% SelectivityNo.% Co% ReSupportConversionC2+CH4CO28121Al2O33387.711.40.93012—TiO2*1187.611.80.631121TiO2*1786.512.80.73212—TiO2**1187.611.70.733121TiO2**1785.813.50.7*support calcined at 500° C.**support calcined at 600° C.
Based on the above disclosure, the person skilled in the art would clearly deduce that TiO2 should be avoided as catalyst carrier for rhenium/cobalt combinations in favour of Al2O3.
Fischer-Tropsch synthesis of hydrocarbons produces a number of by-products such as carbon dioxide, water and gaseous C1-4 hydrocarbons.
As well as improving % CO conversion, it is of prime importance to be able to adjust the product slate in any given Fischer-Tropsch reaction, to satisfy individual requirements such as increased % C5+ selectivity and reduced CH4 and CO2 production.
It is highly desirable to reduce the amount of carbon dioxide evolved during Fischer-Tropsch synthesis of hydrocarbons for both economic and environmental reasons. It is particularly desirable to restrict the level of carbon dioxide by-product in such process to less than 2% v/v, preferably less than 1% v/v.
Of prime importance is that any methodologies employed for a reduction in carbon dioxide selectivity in Fischer-Tropsch synthesis, do not cause a concomitant reduction in C5+ hydrocarbon selectivity.
It can be seen from Table 1, that whilst the addition rhenium to a cobalt catalyst on titania gives a modest increase in activity from 11% carbon monoxide conversion to 17% carbon monoxide conversion, the C2+ selectivity is reduced and the CO2 selectivity is equal or increased compared to the corresponding unpromoted catalyst.
WO-A-97/00231 relates to a catalyst comprising cobalt and manganese and/or vanadium supported on a carrier wherein the cobalt:(manganese+vanadium) atomic ratio is at least 12:1.
Said catalyst exhibits higher C5+ selectivity and higher activity when used in the Fischer-Tropsch synthesis of hydrocarbons, compared to catalysts containing cobalt only, or containing a relatively higher amount of manganese and/or vanadium. Preferred carriers include titania, zirconia and mixtures thereof.
It is highly desirable not only to increase further the C5+ selectivity of such cobalt manganese catalysts, but also to reduce their carbon dioxide selectivity.
It has now been surprisingly found that the addition of small quantities of rhenium and/or ruthenium to cobalt-manganese catalyst compositions can not only cause reductions in carbon dioxide selectivity, but can also have dramatic effects on the product slate obtained from FT hydrocarbon synthesis reactions.